When a structure, such as a main wing, composing an airframe of an aircraft is struck by lightning, a high current flows. A typical structure of an aircraft is one in which plural panels (skins) are coupled to each other. Therefore, when an aircraft is struck by lightning, a spark may occur in a connection part of panels. Occurrence of a spark may cause scattering of molten materials due to an increase in temperature. Meanwhile, since main planes of an aircraft also generally serve as fuel tanks, it is necessary to take measures to prevent molten materials, caused by a spark, from contacting with a combustible fuel vapor.
Thus, a technique to couple panels by sandwiching a bridge layer having conductivity has been devised (for example, refer to Japanese Patent Application Publication JP H02-225200 A). According to this technique, a current flowing in a panel due to a stroke of lightning can be made to flow in another panel through a bridge layer. That is, electrical continuity between panels can be secured by a bridge layer.
Furthermore, a technique to prevent current from flowing in fasteners, for coupling a panel to a structural material, by applying an insulating sealant to the fasteners or using insulating washers and/or insulating spacers has also been devised (for example, refer to Japanese Patent Application Publication JP 2009-227166 A). In the case of applying an insulating sealant to fasteners, it is necessary to make a clearance gap for forming an insulating sealant layer between each fastener and each insertion hole. A connecting method by fitting a fastener in a state where a clearance has been formed between the fastener and an insertion hole as described above is called clearance fit.
In the case of attaching a fastener by the clearance fit, a distance between the fastener and the insertion hole is not necessarily constant. Therefore, even when the insulation property of fasteners is improved using insulating washers and/or insulating spacers, it is still difficult to certainly avoid a possibility that a spark occurs between the fasteners and a panel.
Thus, a technique to couple panels to each other, with making fasteners contact with insertion holes, has been devised (for example, refer to Japanese Patent Application Publication JP 2012-192752 A). A connecting method by fitting a fastener in a state where the fastener contacts with an insertion hole is called interference fit or transition fit. In this method, uninsulated fasteners are used as conductors for current flowing between panels to be connection targets.
However, when a method of coupling panels by fasteners to be the interference fit is adopted, the panels may be damaged due to the contact with the fasteners. In particular, since the use of a composite material as a material of a panel has advanced in recent years, it is an important problem to prevent damage to a composite panel caused by contact between the composite panel and metallic fasteners.
When a panel is opened and closed like a case of an access panel of a fuel tank, fasteners are attached and detached. Therefore, to prevent damage to a panel due to attachment and detachment of fasteners is a more important problem. Furthermore, in the case of interference fit, not only an access panel is damaged but fasteners also wear whenever the panel is opened and closed. Thereby, it is often necessary to exchange the fasteners for new fasteners in addition to repairing and exchanging the panel. Moreover, in the case of interference fit, work of attaching and detaching fasteners takes more time and effort compared to the case of the clearance fit in which a space exists between a panel and each fastener. Furthermore, in the case of interference fit, there is disadvantage that a tolerance in manufacturing error of insertion holes formed on panels to be connection targets becomes small.
As described above, when the interference fit is adopted as a method of coupling fasteners, disadvantage, such as damage to a panel, wear of fasteners, increase in labor at work of attaching and detaching fasteners, and decrease in degrees of freedom in a manufacturing error, arises. Meanwhile, when the clearance fit is adopted as a method of coupling fasteners, there is a problem that a risk of occurrence of a spark arises at the time of a stroke of lightning since non-uniformity exists in clearances between fasteners and a panel as described above.
Thus, an object of the present invention is to allow coupling panels for an aircraft under more satisfactory conditions, with preventing a spark at the time of a stroke of lightning.